1. Field of the Invention
The present invention relates to microwave ovens with a capability of convection heating by circulation of hot air within the microwave cavity.
2. Description of the Related Art
Cooking appliances arranged for both microwave heating and convection heating by hot-air circulation are known. Such appliances can generally operate in microwave mode, where microwave heating is effected, or in convection mode, where heating by hot-air circulation is effected. It is also possible to use a combination of the two modes, in which microwaves assist the hot-air heating process.
In a fan-assisted convection heating oven, it is generally desired to have a uniform distribution of hot air within the cooking chamber in order to promote even heating of foodstuff placed therein. Typically, a heater and a fan are placed behind the rear wall of the microwave cooking cavity in a dedicated compartment. Air is drawn by the fan from the cooking cavity into the compartment through perforations in the rear wall. The fan is placed directly behind the perforations. The air sucked in from the cooking cavity is then blown radially out from the fan and passes the heater to reach an elevated temperature, and is thereupon reintroduced into the cooking cavity through additional perforations at the outer edges of the rear cavity wall. In this way, air is cycled through the cavity and the dedicated heater compartment to provide convection heating for the foodstuff placed in the oven.
The perforations in the rear wall of the cavity are typically sufficiently small in order to prevent leakage of microwave energy from the microwave cooking cavity.
However, these perforations have the potential to cause an obstacle for the circulating air, leading to a lower flow of air than what is desired. To overcome this, the air flow (fan speed) or the air temperature must be increased, which unfortunately may lead to over-heating or drying of foodstuffs or portions thereof in the vicinity of the inlet perforations.
The inventors of the present invention have also identified additional problems with the prior art. In the arrangement as described above, it is believed that the hot air injected into the cooking cavity through the perforation at the outer edges of the rear wall may exhibit a whirl. In other words, it appears there is a tangential component in the air flow (i.e. the air does not flow perpendicularly towards the front of the cavity) which may cause regions of different temperature in the cavity. This whirl is a residue of the fan action that is communicated into the cooking cavity through the perforations. As a result, due to the whirl, the hot air might not reach all parts of the cooking cavity, which could result in uneven heating. Accordingly, there is a need for a cooking appliance which improves or helps overcome the known drawbacks associated with the prior art.